The present invention relates to a method of forming a graphene layer on a non-hexagonal lattice, and a structure obtained by the same.
Graphene is a structure consisting of carbon as a two-dimensional sheet. A graphene monolayer has a thickness of about 0.34 nm, i.e., which is approximately the atomic diameter of a single carbon atom. A graphene layer can exist as a monolayer of a two-dimensional sheet. Alternately, a graphene layer can exist as a stack of a plurality of two-dimensional monolayers of carbon, which do not exceed more than 10 monolayers and is typically limited to less than 5 monolayers. Graphene provides excellent in-plane conductivity. Semiconductor devices employing graphene have been suggested in the art to provide high-density and high-switching-speed semiconductor circuits. Carbon atoms are arranged in a two-dimensional honeycomb crystal lattice in which each carbon-carbon bond has a length of about 0.142 nm.
A graphene layer may be grown by direct epitaxial deposition of carbon atoms on, i.e., addition of carbon atoms onto the surface of, a surface of a single crystalline silicon carbide (SiC) substrate having a hexagonal symmetry such as a (0001) surface of alpha silicon carbide. Alternately, graphene can be grown by heating a hexagonal surface of a single-crystalline silicon carbide material at a temperature greater than 1,100° C. in vacuum or in a non-vacuum ambient. The process of forming graphene by a high temperature anneal is not an additive epitaxy in its strict sense, but is actually a subtractive process in which silicon atoms on a hexagonal surface of a silicon carbide crystal are evaporated during the anneal. The remaining monolayer of carbon forms graphene with an epitaxial relation to the SiC surface.
Alpha silicon carbide has a hexagonal crystal structure, and beta silicon carbide has a cubic crystal structure of zinc blende type. FIG. 1 schematically shows the crystallographic structure of alpha silicon carbide. A (0001) surface is perpendicular to the c-axis and atoms in the (0001) surface are arranged in a pattern having a hexagonal symmetry. The plane in which the (0001) surface is located is referred to as a C plane. A (1102) surface of alpha silicon carbide crystal has a cubic symmetry, and does not have a hexagonal symmetry. The lack of hexagonal symmetry in the (1102) surface of silicon carbide has been considered a detriment to formation of any epitaxial hexagonal structure thereupon.
Silicon carbide substrates having a (0001) surface orientation are not commercially available at a diameter greater than 4 (or 5) inches at the present time. Such unavailability of silicon carbide (SiC) substrates currently makes it impossible to provide a 200 mm substrate or a 300 mm substrate containing a graphene layer. Thus, formation of graphene by epitaxy on a hexagonal surface of a silicon carbide crystal is limited to epitaxial deposition process performed directly on commercially available silicon carbide substrates having a hexagonal symmetry, but cannot be performed on substrates having a diameter of 6 inches or greater.
Prior art methods for forming a graphene layer on a single crystalline silicon carbide (SiC) substrate either by deposition or by reduction of silicon has required a surface having a hexagonal lattice symmetry, which is the same type of lattice symmetry as the in-plane symmetry of the graphene layer. In other words, formation of a graphene layer on a silicon carbide single crystal has required a (0001) surface of alpha silicon carbide having a hexagonal crystal structure.
In order to effect epitaxial growth of graphene or formation of graphene by an anneal, prior art methods require a surface having a hexagonal symmetry so that the graphene to be formed can be aligned to the underlying hexagonal symmetry. Specifically, prior art methods require that a (0001) surface of alpha silicon carbide be provided for either deposition or reduction of silicon in order to form a graphene layer on a surface of silicon carbide.